Theory of
Sexual Selection

The obvious starting point for any discussion of sexual selection is to note
that one of the most common differences between the sexes in sexually
reproducing species is that males often have bright colours while their female
counterparts do not. The archetype example of this is the peacocks tail.
Generations of biologists and evolutionary theorists have been intrigued by the
question; how and why did the gaudy and cumbersome plumage of the peacock
evolve? Charles Darwin himself suggested that the answer to this question may
lie in female choice in his book The Descent of Man and Selection in
Relation to Sex and this is the basic premise of the theory of sexual
selection.

For the most part we expect to be able to explain most of the
characteristics of an organism as adaptations; that is we assume that
they have been designed by natural selection  the
filter which only allows the genes of successful individuals to reach the next
generation. So, when we observe that an organism is well suited to its
environment this is the explanation for how its attributes became honed to that
particular environment. In the case of animals this argument applies not only
to physical attributes, but also to behaviour. This idea was explained
first by Darwin in his revolutionary The Origin of Species. However, if
this is the case should we not be able to explain the bright colours of
peacocks in terms of some survival advantage such as its use as camouflage?
Many theorists have attempted to do just this, but the theory of sexual
selection suggests an alternative.

First we note that only the genes of those individuals who successfully
reproduce can be selected  this depends on both the ability of the
individual to survive lifes exigencies for long enough to reach
reproductive age and the ability to attract a mate. Thus we would expect
that the behaviour and appearance of an animal is adapted not only to helping
it survive, but also to helping it acquire the largest number or the highest
quality mates. Returning to our example of the peacock and assuming that some
time in the past the peacocks male ancestors were drab colours like their
female counterparts. What if, at some point in evolutionary history, there was
a tendency for peahens to prefer slightly brighter coloured males? The gaudier
males would then be more likely to find mates than their less gaudy rivals
which would in turn lead to a tendency for brighter males in the next
generation. This would provide a selective advantage for the brightest males
which could lead to a runaway process where males would get brighter and
brighter with each generation. Even females who bucked the trend would be at a
disadvantage for, if they chose to mate with plainer males, they risk producing
drab sons who would be less likely to be picked as mates in their turn. Thus
selection pushes both sexes into producing and choosing ever-brighter males.
Only when the disadvantages in terms of surviving for long enough to reproduce
outweigh the advantages in terms of being selected as a mate would this process
cease.

This idea of female preference driving the evolution of male gaudiness has
been demonstrated to the satisfaction of most. However, it begs the question of
why preferences begin in the first place. Echoing Darwins earlier
argument Ronald Fisher argued in the 1930s that female preference for bright
colours was completely arbitrary, and there are those that still hold to this
idea, often referred to as Fisherians. There are those who disagree with this
theory and argue rather that the peacocks plumage (or, equivalently, the
length of a swallows tail, the sweetness of a blackbirds song) is a
signal that the male has high quality genes, and thus that the females
selection is based on rational criteria (because such behaviour has been
selected for  this does not mean that the females are rational or even
know that they are choosing). Fisher rather favoured the idea that initially a
preference may be based on some indication of health and vigour, but that once
the selection process had begun it could run away and result in a preference
for something which was unrelated to good genes. Another insight comes from
Amotz Zahavi, who suggested that the fact that the tail is a handicap is itself
a signal showing how strong and healthy a particular male is; he can survive in
spite of his handicap. Regardless of these disagreements, the theory of sexual
selection, based on the idea of female choice drives the evolution of
particular traits, is accepted my most theorists.

Before we go on to look at the relevance of sexual selection theory to human
evolution, we need to address the question; why female as opposed to male
choice? In order to explore this, it is worth introducing the term parental
investment. Parental investment (PI), an idea first introduced by R. L.
Trivers, is defined as any investment by a parent in one of her (his) offspring
that increases the chance that the offspring will survive at the expense of
that parents ability to invest in any other offspring (alive or yet to be
born). PI then includes the provision of a wide range of resources such as
food, energy and time expended obtaining food and maintaining the home or nest;
time spent teaching children and risks taken to protect young. In terms of PI,
there is a fundamental asymmetry between the sexes  females have an
initial investment in their offspring far greater than that of males because
female gametes (eggs) are much more costly to produce than those of males
(sperm). This means that a female can have only a limited number of offspring,
whereas a male can have a virtually unlimited number, provided that he can find
females willing to mate with him. Thus females generally need to be much
choosier about who they mate with. The criteria for what constitutes a good
choice of male will vary considerably from species to species, but the basic
point about female choice remains.

No discussion of peacocks would be complete without some mention of the
lek paradox. At mating time peacocks gather in large congregations
called leks at which they strut their stuff and show off their wares to
peahens. Not all the peacocks get to mate and some get to mate several times.
The paradox arises because, if all of the females choose to mate with the same
few males  those with the best genes, then there will be much
less genetic variety in the population in the next generation, and over a
number of generations we might expect this to lead to no variety, making it
impossible to sustain any choice. Many solutions to this paradox have been
suggested, notably that of Hamilton and Zuk, who followed through a particular
train of thought regarding sexual reproduction itself. In the 1920s and 30s the
modern synthesis of the idea of natural selection with the science
of genetics (which began with the work of Gregor Mendel) took place. In the
1970s Richard Dawkins popularised one interpretation of the synthesis in his
famous book The Selfish Gene. The selfish gene hypothesis is that the
gene, as opposed to the individual organism or group of organisms is the
level at which natural selection takes place, since only genes are passed into
the next generation (replicated). This raised the question; why do so many
organisms reproduce sexually rather than asexually when asexual reproduction
clearly gets more of an individuals genes into the next generation than
sexual reproduction? This was answered, at least in part by Bill Hamilton, who
argues that sex is an important part of the struggle against disease 
mixing genes provides greater genetic variety and thus greater chance of
resistance to particular diseases. This is where we come back to Hamilton and
Zuks hypothesis mentioned above. They argued that the tail tells a tale
about the health of a male! The state of a peacocks plumage tells a
peahen much about whether or not the male has blood parasites. Thus those males
with the best plumage will not necessarily be the descendants of those with the
finest tails of the last generation, solving the lek paradox.

Now we come to the question of how sexual selection is relevant to the human
mind. Three million years ago our ancestor the upright ape Australopithecus
afarensis, also known as Lucy, had a brain size of about 400cc. Modern
humans have a brain that is a remarkable 3½ times that size, at 1400cc.
This inordinately large brain is very costly to run; the brain consumes 18% of
our energy expenditure. From a Darwinian perspective this suggests that there
must have been significant and immediate advantages to possessing a larger
brain which outweighed the expense.

Thus we come to the question; why did we need to become so intelligent? The
answers to this question are many and varied; Was it that accumulated knowledge
played a crucial role in enabling humans to develop a rich, varied diet, which
in turn required the capacity for language and for a large memory? Or was it
that a sexual preference for juvenile features drove us towards prolonged
retention of such features (neoteny) which in turn allowed the development of a
larger brain as a secondary effect of a longer period of growth?

One idea that has become influential in the last few years is known as the
Machiavellian hypothesis, which has its origins in the work of Richard
Alexander in the 1970s. He suggested that the main evolutionary pressure for
human beings to increase in intelligence was competition with other people, in
particular, sexual competition between individuals of the same sex. The
argument runs thus; the primary function of most animal communication is to
manipulate others, not just to impart information; the ability to deceive and
to detect deception in others is highly important for many social animals; thus
it is reasonable to assume that this principle underlies the evolution of our
highly developed communicative ability. What is more, in order to be really
convincing to others in our deception it became necessary to develop the
ability for individuals to deceive themselves; Robert Trivers has argued that
this may be why we developed a subconscious. In a similar vein Nicholas
Humphrey argued that the need to guess the likely actions of other individuals
required us to develop the capability to imagine what is in others minds;
a theory of mind, which was a key factor in the development of
self-consciousness.

Matt Ridley does not find any of these explanations satisfactory as they
still beg the question why humans, and not other apes, evolved such a large
brain. Following Geoffrey Miller he suggests that, in fact, only runaway
sexual selection as described by Fisher is sufficient to explain the
huge increase in brain size. Being intelligent, witty and entertaining was sexy
to our ancestors! This hypothesis requires only that there was an initial
preference for more intelligent mates that drove the process from then on. This
helps to explain why only humans developed in this way. First, because the
initial preference was quite arbitrary and only came about by chance and we can
thus assume that an equivalent preference never arose in the other apes.
Second, because the human mating system is unique among apes in that it is
characterised primarily by monogamous pair bonding (with occasional polygamy)
and shared parental effort in child rearing. This leads to a whole set of
tendencies untypical of apes such as the preference of males for young or
youthful-looking mates and the preference of females for high status, often
older, men.

Underlying all of these theories is the hypothesis that, because we have not
evolved much in the last hundred thousand years, and because we evolved in a
particular environment (sometimes described as the environment of evolutionary
adaptedness or EEA which is usually assumed to be the African savannahs) living
in a particular way (as social usually monogamous, occasionally polygamous
hunter-gatherers) our minds should be adapted to that way of life.

It is worth noting that this is quite different from the approach taken by
sociobiology in the 1970s, which attempted to explain modern human behaviour as
adaptive. Modern theorists, in particular the proponents of evolutionary
psychology (EP), rather attempt to explain the human mind in terms of
tendencies which developed in the EEA.

As Matt Ridley puts it in The Red Queen:

There has been no genetic change since we were
hunter-gatherers, but deep in the mind of modern man is a simple
hunter-gatherer rule: strive to acquire power and use it to lure women who will
bear heirs; strive to acquire wealth and use it to buy affairs with other
mens wives who will bear bastards . . . Wealth and power are means to
women; women are means to genetic eternity.

Likewise, deep in the mind of modern woman is the same
hunter-gatherer calculator, too recently evolved to have changed much: strive
to acquire a provider husband who will invest food and care in your children;
strive to find a lover who can give those children first-class genes. Only if
she is very lucky will they both be the same man . . . Men are to be exploited
as providers of parental care, wealth and genes.

However, Ridley tells us that we need not interpret any of these arguments
in a deterministic way; none of this denies our free will. To say that a
tendency is in our nature is not to say that we cannot overcome that tendency.
What is more, the fact that we are so versatile and plastic in terms of
behaviour actually depends on us having more instincts, not fewer; the language
instinct being a prime example. The vocabulary of language may be infinitely
plastic, but our ability to learn languages depends heavily on our hard-wired
ability to form generalised rules based on what we hear; to infer the rules of
grammar.